The present invention relates to a collimator, specifically an X-ray collimator formed of three or more perforated collimator plates.
Collimators are employed in X-ray equipment and in order radiography equipment utilizing radiation with short wavelength. The purpose of the collimator is to prevent the radiation which is scattered from the object under examination, the so-called stray radiation, from affecting the film.
Stray radiation deteriorates the quality of a radiograph by weakening both contrast and sharpness. The thicker the object under examination, the more there occurs stray radiation. The amount of scattered radiation, for example in patient radiography, often surmounts the amount of unscattered radiation. The amount of scattered radiation can even be of multiple quantity in comparison with the amount of unscattered radiation.
In the prior art, collimators are known which consists of one or two collimator plates, which are placed near the film cassette. These plates are generally composed of narrow, upright bands of leaden plate, and in between the plates there is located a layer of plastic or aluminium, which layer easily transmits X-ray radiation. Depending on the particular application, the thickness of the collimator plates as well as the number of slits per length unit can alternate.
Among the drawbacks of the above described slit collimator the most conspicuous is the fact that it does not obstruct stray radiation directed parallel to the slits. The collimator functions best in the direction perpendicular to the slits. Even such scattered X-ray quantums which are quite oblique can affect the film, if they happen to pass through the slit. It is true that this drawback can be avoided by constructing a double-layer collimator plate with two slit collimators of the above described type positioned crosswise on top of each other. But because the structure of only one slit collimator plate is fairly complicated, and consequently the price is high, it is clear that these problems become with a double plate. Moreover, the double collimator plate absorbs an increased amount of the X-ray radiation needed in the radiography operation.
The German Patent Publication DE-OS No. 2 031 203 introduces a collimator plate which is placed near the film cassette and which is provided with perforations at regular intervals or at random intervals with certain limitations. These perforations obstruct stray radiation from affecting the film better than the slits. The drawback of this collimator arrangement is that only one collimator plate is used in the neighbourhood of the film cassette. Consequently the scattered radiation is not effectively eliminated in this arrangement either.